Why do we use "&"?

[SWEngineer]

At the following web page: http://web.mit.edu/qt-dynamic/www/tu...torial-t2.html

In:

QObject::connect(&quit, SIGNAL(clicked()), &app, SLOT(quit()));

QObject::connect(&quit, SIGNAL(clicked()), &app, SLOT(quit()));

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Why is & used with quit and app? Cannot we use quit and app without &?

Thanks.